The U-turn throws into chaos Mr Clarke’s shake-up of the criminal justice system.

Judges are currently under strict instructions on which murders merit a 'whole life' sentence behind bars and which should be imprisoned for 15, 25 or 30 years. Those who kill police get a minimum of 30 years.

But policy experts in Downing Street and the Ministry of Justice
spent months working on plans to rewrite the section of the law that
spells out minimum sentences.

Asked to explain the
consequences on Tuesday, Mr Clarke dismissed the current rules as
'complete nonsense'.

'At the moment, if you murder me you’ll be punished more if you use a
knife than if you strangle me painfully. I don’t think you should be too
prescriptive.

'It is absurd to say the judge needs a statute to say what method of
murder is more serious than another method.

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'We
do not need to tell judges that murder is a serious offence. They are
perfectly capable of setting a minimum term.'

But police officers, bereaved families and knife campaigners, were
outraged by his proposals and victims groups announced plans for a
demonstration.

Before the U-turn was announced, Simon Reed,
of the Police Federation, had said: 'Rather than diluting this 30-year
rule we should be making sure police killers actually spend life behind
bars.

Judges to decide: Justice Secretary Ken Clarke said on Tuesday that there was no need for a minimum sentence

Tory MPs openly denounced the plans and the ConservativeHome website, an indicator of the views of the Tory grassroots, was inundated with complaints.

But as Mr Cameron returned from Afghanistan yesterday, his advisers sought to clarify the Government's position, saying the minimum sentences 'were never going to be scrapped'.

Instead, a Green Paper will see Schedule 21 of the 2003 Criminal Justice Act, which spells out the minimum sentences, rewritten, they insisted.

'We will look to simplify it, to make it clearer, easier to use, and more readily understood by victims' families and the public,' a Downing Street spokesman said.

'This Government has absolutely no intention whatsoever of reducing sentences for murder.

'We will never abolish the mandatory life sentence, or seek any general reduction in minimum terms imposed for murder.'

No 10 intervened amid concerns that the plans would undermine Mr Cameron's election pledge to get tough on knife-wielding criminals, which included plans to jail anyone found guilty of crimes involving a knife.

Sources close to Mr Cameron said Ben's Law, named after 16-year-old Ben Kinsella who died at knife point in 2008, which means knife killers must serve 25 years, 'will definitely not be scrapped'.

One said: 'The idea that we were going to end mandatory sentences is for the birds. On murder, nothing will change.'

It is the second time this year that Mr Cameron has had to intervene to overrule the Ministry of Justice.

No 10 officials disciplined Justice Minister Crispin Blunt after he announced that taxpayers would have to fund prison Halloween parties for killers and paedophiles.

Labour leader Ed Miliband said: 'They’re clearly in chaos over this and it’s a shambolic way to make policy.'